The most common construction for photo glossy inkjet receivers is based on polymeric (i.e., resin-type) coatings rather than microporous coatings. In such designs, the polymeric receiver swells to absorb the ink solvent (vehicle), and the inkjet dyes are fixed by cationic sites on the binder or receiving layer additives. While these resin-coated receivers offer many advantages, they also have some common shortcomings, including slow dry times, tackiness under high humidity conditions, and coating solubility or softening when exposed to water. Polyvinyl alcohol (“PVOH”; sometimes called “PVA”), poly (vinylpyrrolidone) (“PVP”), and gelatin are among the most common water-soluble polymers used in their construction. Among these polymers, gelatin is of particular interest in the present invention. Gelatin, also known as collagen, is a unique natural polymeric material containing hydroxyl, carboxyl, and amine functional groups. Gelatin exhibits good absorption of water and alcohols commonly used as ink solvents (e.g. ethylene glycol), forms a clear film, has cationic sites for dye fixation, and provides good light-fastness. Additionally, the carboxyl groups provide ample sites for cross-linking with other materials and are reported to provide a buffering action against acidity induced tonal distortions.
Glossy inkjet media based on microporous coatings eliminate or reduce many of the shortcomings of the polymeric coated media with regard to dry time, tackiness, and water sensitivity. However, many suffer from significantly poorer performance in light-fastness and ozone-fastness, and generally have lower gloss and ink density compared to resin coated media. The use of alumina hydrate in microporous inkjet receptive coatings is fairly well known in the art. Early development of pigment based, or microporous, inkjet media focused on silica as the primary pigment. However, alumina hydrate possesses a positive (cationic) surface charge capable of complexing anionic inkjet dyes, in addition to the physical characteristics that make silica so attractive. This combination of properties makes alumina hydrate particularly well-suited among pigments for high quality inkjet applications.
Prior art coatings that use alumina hydrate out of necessity, or at least as a preferred component, rely primarily on PVOH or PVP as a binder. Gelatin and alumina hydrate, particularly alumina hydrate of the boehmite structure, have limited compatibility. The following prior art patents illustrate the limitations in combining alumina hydrate and gelatin in an inkjet coating formulation. U.S. Pat. No. 5,911,855 describes a printing material comprising a support, a dye receiving coating, and an upper coating layer. Gelatin is the primary component of the dye-receiving layer, but this layer is distinct from the porous upper layer, which is composed of boehmite and other water-soluble polymers.
U.S. Pat. No. 5,804,320 discloses a recording medium comprised of an ink-receiving layer composed of a pigment, particularly alumina hydrate, and a heat treated alkali processed gelatin that does not gel at room temperature and which has a weight averaged molecular weight (MW) in the range of 50,000-150,000. The described coating dispersion maintains stable viscosity, but low coating solids are required owing to the high molecular weight of the gelatin.
U.S. Pat. No. 5,738,932 discloses an inkjet recording medium composed of an alumina hydrate and an acid-processed gelatin with MW in the range of 20,000-200,000 or alkali-processed gelatin with MW in the range of 5,000 to 100,000. The gel forming ability of the gelatin and thixotropic nature of the alumina sol are exploited to achieve high coat weight without sagging in a single pass. Even at the lower end of the stated molecular weight ranges, one would expect relatively high viscosity at low coating solids. Owing to the low solids of coatings so constructed (e.g. less than 15% for all examples), preferred coating application methods include kiss coating, extrusion coating, slide coating, and curtain coating. While high-speed coating operations such as blade coating, roll coating, and gravure coating are mentioned as possible coating application methods, one would expect that several coating applications would be necessary to achieve an effective coating thickness.
As further background for the present invention, U.S. Pat. No. 6,194,077 discloses a resin-type, water-fast, ink-receptive material composed of gelatin having a Bloom value of 100 to 300, a water-insoluble quaternary cationic polymer, and a multifunctional cross-linker.